At least nine rodeo horses are dead in Calgary near the end of a 200-kilometre trail ride held as part of the city's famed Stampede.

About 200 horses left the Stampede Ranch near Hanna in east-central Alberta on Tuesday. The horse drive was to celebrate both the Stampede, which starts Friday, and Alberta's provincial centennial.

When they were on a bridge over the Bow River, still flowing high and fast after recent heavy rains, the animals got spooked about 2:30 p.m. local time.

Some animals panicked and leaped into the river about 10 metres below. Three died on impact. The others drowned after they were swept downstream. One was put down later because of its injuries.

A family out for a bike ride witnessed the incident.

"I was hoping the horses could make it out, but after they went underneath the water ... pretty sad," said Kit Kuizer.

His father Eric said: "Initially, I thought it was deliberate. The horses were all of a sudden starting to run and as soon as they hit the bridge, they were going to run to the Stampede. But within three seconds, I realized there was a major problem going on."

His wife Jane thought a nearby train might have spooked the animals.

"The engine got really loud, and it was shunting cars back and forth," she said. "It seems like the horses just took off at that point, because they were really calm (before), actually."

Stampede spokesman Lindsey Galloway said: "It's been a tragedy. It's a sad day for the Stampede. We're very upset about it."

The animals were wild, and moving them poses risks, he said, but added similar drives in 1987 and 2000 were complete successes.

The rest of the horses continued to the Stampede grounds with one incident.  However, a chuckwagon tipped, leaving the driver with an injured groin.

Officials say one horse remains unaccounted for.

The surviving horses will be used as bucking stock in the riding events of the Stampede's world-famous rodeo.

With a report from CTV's Nujma Yaqzan